The demand for wireless communication systems has grown steadily, and is still growing, and a number of technological advancement steps have been taken during this growth. In order to acquire increased system capacity and user bit rate for wireless systems by employing uncorrelated propagation paths for data streams, MIMO (Multiple Input Multiple Output) systems have been considered to constitute a preferred technology for improving the capacity.
MIMO employs a number of separate independent signal paths for data streams, for example by means of several transmitting and receiving antennas. The more signal paths that are available, the more parallel data streams may be transmitted.
Especially at the terminal side there is normally a limited volume available in the terminals used, which generally will lead to a high antenna coupling which will deteriorate the performance of the system by increased correlation between the received or transmitted signals and by decreased signal to noise ratio due to reduced efficiency of the antenna system.
There are several previously known methods to decrease the effects of coupling. According to EP 1349234, a compensation is performed on the signal by means of signal processing. This is disadvantageous, since the coupling still occurs although the coupling effects are compensated for, resulting in undesired power losses.
In general, the signals will also be even more correlated after this compensation, since the isolated antenna patterns are restored. It is a well known fact that the coupling decreases the correlation between the received signals in a Rayleigh scattering environment.
According to J. B. Andersen and H. H. Rasmussen, “Decoupling and de-scattering networks for antennas”, IEEE Trans. on Antennas and Propagation, vol. AP-24, pp. 841-846, 1976, a lossless network is connected between the input ports and antenna ports of a number of antennas. This network has such properties that there is no coupling and scattering between the antennas. There are, as pointed out in the paper, some rather severe limitations. Firstly, the scattering pattern has to equal the transmit pattern, a property that only minimum scattering antennas have. Secondly, all mutual antenna impedances have to be reactive, which means that the distances between the antenna elements have specific values which may not be altered. For example, in a linear array of three monopoles, this condition cannot be fulfilled since pure reactive mutual impedances between the outer elements and between adjacent elements cannot be obtained simultaneously. As a conclusion, this prior art provides a method that only works for certain specific geometries.
Another commonly used technique at the base station to reduce antenna signal correlation is to increase the separation of the antennas, e.g. for receive diversity. This is not practical to implement in a handheld terminal.